1. Field of the Invention
The present invention relates to a liquid-crystal display apparatus of a birefringence-control type.
2. Description of the Related Art
Various liquid-crystal display apparatuses are known, classified in accordance with their operating modes. Among them are: a TN (twisted nematic) type, a DS (dynamic scattering) type, a GH (guest-host effect) type, a DAP (deformation of vertical aligned phases) type, and a thermally writing type. Of these types of the liquid-crystal display apparatuses, the TN type is used in various devices such as wrist watches, hand-held calculators, and measuring devices. The TN liquid-crystal display apparatus has neither an image contrast nor a view angle range great enough to provide a display screen which is as large and can display as much data as is demanded at present. Accordingly, it has been greatly demanded that a liquid-crystal display apparatus of a new operating mode be developed which can have an adequate image contrast and a sufficiently wide view angle.
In recent years, a liquid-crystal display apparatus, which meets this demand, has been invented. This is known as the birefringence-control type. Japanese Patent Disclosure No. 60-107020 discloses an SBE (super-twisted, birefringence effect) liquid-crystal apparatus which is one type of a birefringence-control liquid-crystal display apparatus. The birefringence-control liquid-crystal display apparatus comprises a pair of parallel transparent substrates opposing each other, electrodes formed on the opposing surface of the substrate, at least one of the electrodes being transparent, and a seal member sealing the periphery of the substrates, thus defining a cell. The cell is filled with nematic liquid crystal such as a cyclohexane-based one, an ester-based one, a biphenyl-based one, or a pyrimidine-based one. The nematic liquid crystal contains chiral agent, which twists the axes of the liquid-crystal molecules by 180.degree. to 360.degree., from one substrate to the other substrate. The axes of the liquid-crystal molecules are pre-tilted at an angle greater than 5.degree. to the surfaces of the substrates, by means of the two crystal-orientating layers formed on the opposing surfaces of the substrates.
In the case of an SBE liquid-crystal display apparatus wherein the axes of the liquid-crystal molecules are twisted by 270.degree., two polarizers are arranged on the outer sides of the substrates, which face away from said opposing surfaces. The structure considered best suited for this SBE liquid-crystal display apparatus is two-fold. First, one polarizer (hereinafter called "front polarizer"), which is arranged on the front side of the liquid crystal cell, has a light-transmitting axis inclined clockwise at about 30.degree. with respect to the direction in which the liquid-crystal molecules are orientated on the front side substrate. Secondly, the other polarizer (hereinafter called "rear polarizer"), which is arranged on the rear side of the liquid crystal cell, has a light-transmitting axis inclined either counterclockwise at about 30.degree. or clockwise at about 60.degree., to the direction in which the liquid-crystal molecules are orientated on the rear side substrate. When the light-transmitting axis of the rear polarizer is inclined counterclockwise at about 30.degree. with respect to the molecule-orientating direction of the rear side substrate, the liquid crystal assumes yellow mode, i.e., a bright yellow display is obtained in a non-selective state, and a black display is obtained in a selective state. On the other hand, when the light-transmitting axis of the rear polarizer is inclined clockwise at about 60.degree. with respect to the molecule-orientating direction of the rear side substrate, the liquid crystal assumes blue mode, i.e., a deep blue display is obtained in a non-selective state and a transparent display is obtained in a selective state. In any other birefringence-control liquid-crystal display apparatus, the background of the display screen is not achromatic color.
The inventors hereof have proposed that the background of the display screen be colorless. (See
Japanese Patent Disclosure No. 1-514). More precisely, the inventors invented a SBE liquid-crystal display apparatus, wherein the value of R ranges from 0.3 to 0.7, R being .DELTA.n.multidot.d.multidot.cos.sup.2 .theta., where d is the thickness of the layer of the nematic liquid crystal, .DELTA.n is the optical anisotropy of the nematic liquid crystal, and .theta. is the pre-tilt angle of the liquid-crystal molecules. Because of the value of R, falling within this specific range, both the spectrum in the field-on state and the field-off state are almost horizontally linear, whereby the apparatus can perform a high-quality monochrome display.
However, even if the value of R falls within the range of 0.3 to 0.7, the liquid-crystal display apparatus cannot have adequate image contrast, sufficient whiteness, or sufficient transmittance. When the apparatus is used as a light-transmission type and back-lighted, its display assumes some color if adequate contrast is maintained, and conversely, its display image fails to have adequate contrast if satisfactory whiteness is maintained.